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# AS level Hess's law - help please! watch

So I'll explain what I do understand:
-DHr= DH(products) - DH(reactants)
-Exo= negative and endo= positive
-I also know which way the arrows go for combustion and formation

I can work out the DH's of the different parts of the equation but get stuck when I have to figure out what to add and take-away from what. Is it different for combustion and formation?

For example: (Sorry if it's hard to read!)
Calculate the enthalpy change of the combustion of ethyne. Use the average bond enthalpies in the table below.

Equation:
H-C(triple bond)C-H + 2 1/2 O2 ---> H2O + 2 CO2

Table:
C-H = +415
C (triple bond) C = +837
O (double bond) O = +498
C (double bond) O = +805
O-H = +464

So I worked out this...

(415x2) + 837 = +1667
(498x2) + 249 = +1245
So DH(reactants) = +2912

464x2 = +928
(805x2) x2= +1610
So DH(products) = +3220

Now I'm stuck! Where do these values fit into the Hess's law diagram and what do I add/take-away?

Thank you so much if you can help and maybe explain it better than my teacher!
2. Once you have all the bond values given, you don't need to use Hess's law , just use: sigma(bondbreaking) - sigma(bondforming)
3. (Original post by WildBerry)
Once you have all the bond values given, you don't need to use Hess's law , just use: sigma(bondbreaking) - sigma(bondforming)
Seriously? That just made my life A LOT easier! Thank you sooooo much!
Seriously? That just made my life A LOT easier! Thank you sooooo much!
TBF this IS a statement of Hess' law...
5. (Original post by charco)
TBF this IS a statement of Hess' law...
Yeah, tbh I don't even think it's worth using!
6. (Original post by WildBerry)
Once you have all the bond values given, you don't need to use Hess's law , just use: sigma(bondbreaking) - sigma(bondforming)
However, the above is not sufficient for A2 (if youre planning to do A2)
You will need to be able to draw a cycle! I was taught to use "sigma(bond breaking - bond forming) when using detla Hf values, as using it for delta Hc values, although not wrong, you may get no credit for it at A2, so it may be best to learn how to do the cycle now.

Hope this helps
7. Not sure if this helps, but if you have the diagram with all three reactancts, write an equation with the arrows going clockwise on one side of the = sign and the arrows going anticlockwise on the other. So you get DH1 + DH2 = DH3 or something like that, two will be known one won't. This should make it a little easier.

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